When one is being given deep throat (hence throat) and at the point of ejaculation unleashes his gear all over the recipients face (cockney rhyming slang for face is boat race). Thus resulting in a bit of throat and boat.

Lee: I copped for some dirty bird last night!!
Ian: Did you get a shag?
Lee: No, but I got a bit of throat and boat...which was rather pleasant!!!
Ian: The dirty bitch!!!